"How is the day?"
Translation:Habari ya leo?
'za' is plural, 'ya' is singular. According to noun class, other variants you'll learn include 'wa,' 'cha,' 'vya,' etc. -- all of which mean 'of.' In this sentence, the literal translation is "News of today?" The plural is used because it is assumed you have more than one piece of news (habari) to share.
Siku is the noun for day, while leo is the adverb today. So if you are talking about a day in any other way than what's going on today, you would use siku. In this sentence you could use both. Compare the English phrases "News of the day?" and "News of today?", which are both perfectly usable, and so also in Swahili.